1. Field of the Invention
This invention relates to a process for the preparation of polyamides. More specifically, the invention relates to a continuous process for preparing polyamides by reaction of omega-aminonitriles in a countercurrent multistage reactor swept with steam.
2. Description of the Prior Art
Polyamides are conventionally prepared by the condensation polymerization of a diacid, such as adipic acid, and a diamine such as hexamethylene diamine, or by the polymerization of lactams such as .epsilon.-caprolactam. Other processes are known which involve preparation of polyamides by reaction of omega-aminonitriles with water. For example, Greenwalt U.S. Pat. No. 2,245,129 discloses preparation of polyamides by heating omega-aminonitriles in the presence of water in a two step process. In the first step, the reaction mixture is heated in a closed reaction vessel to form a low molecular weight polyamide. In the second step, the liberated ammonia and excess water is removed at atmospheric or reduced pressure with simultaneous and/or subsequent heating of the polyamide to increase the molecular weight of the polyamide. Curatolo et al. U.S. Pat. No. 4,568,736 discloses using oxygen-containing phosphorus compounds as catalysts during the reaction of omega-aminonitriles to form polyamides. Marks U.S. Pat. No. 5,109,104 discloses the batch polymerization of omega-aminonitriles to produce high quality polyamides. In international patent application PCT/EP/97/0460. corresponding to international publication number WO 98/0889. a process for producing a polyamide by reacting an aminonitrile with water at sequential staged temperatures and staged pressures is disclosed. It is also known to produce polyamide within a continuous reactive distillation column involving significant rectification of an aqueous salt solution of a diamine and dicarboxylic acid as disclosed for example in U.S. Pat. No. 3,900,480. In a recent international patent application PCT/EP/98/08239, corresponding to international publication number WO 99/43732 published after the priority date of the present application, the reaction of an aminonitrile with water by reactive distillation is taught.
Most of the processes that have been disclosed in the art have been batch, with attendant potential disadvantages of high operating labor, within-batch final product non-uniformity, and batch-to-batch product non-uniformity. The current invention provides a continuous process for preparing polyamides from omega-aminonitriles which overcomes these problems.